1. Field of the Invention
The present invention relates to a method of manufacturing an electrolytic capacitor and to an electrolytic capacitor obtained by the manufacturing method. More specifically, the invention relates to a method of manufacturing a wound electrolytic capacitor and an electrolytic capacitor obtained by the manufacturing method.
2. Description of the Background Art
As digitization of electronic equipment proceeds, it has become required that capacitors used therefor have a smaller size, a larger capacitance and a smaller Equivalent Series Resistance (hereinafter abbreviated as ESR) in a high-frequency range.
As capacitors for a high-frequency range, plastic film capacitors and multilayer ceramic capacitors for example have conventionally been used frequently. Such capacitors, however, have a relatively small capacitance.
An example of a small-sized, large-capacitance and low-ESR capacitor is an electrolytic capacitor having, as a cathode material, such an electron conducting material as manganese dioxide or TCNQ complex salt. Here, TCNQ refers to 7,7,8,8-tetracyanoquinodimethane. Further, an electrolytic capacitor for which such an electrically conductive polymer as polypyrrole, polythiophene, polyfuran or polyaniline is used is also a promising capacitor. For example, Japanese Patent Laying-Open No. 02-015611 discloses an electrolytic capacitor including a specific polythiophene as a solid electrolyte.
In manufacturing an electrolytic capacitor having such an electrically conductive polymer as described above as a cathode material, on a surface of a sintered anode or anode foil made of such a valve metal as aluminum or tantalum for example, a chemical conversion coating film, an electrically conductive polymer layer, a graphite layer and a silver paint layer are formed successively, to which a cathode lead is connected by means of an electrically conductive adhesive or the like. This method, namely the cathode draw-out method is considerably complicated as compared with the method impregnating, with an electrolytic solution, a capacitor element including an anode foil on which a chemical conversion coating film is formed and an opposite cathode foil that are wound or rolled with a separator therebetween, which is a method of manufacturing a so-called wound electrolytic capacitor. Further, the cathode draw-out method tends to provide a larger ESR as compared with a wound electrolytic capacitor using the opposite cathode foil.
In manufacturing a wound electrolytic capacitor, the electrically conductive polymer layer as described above can be formed using, for example, electrolytic polymerization method or vapor phase polymerization method. However, it is operationally complicated and difficult to form an electrically conductive polymer layer within a wound capacitor element using the electrolytic polymerization or vapor phase polymerization method. A possible method forms, on an anode foil, a chemical conversion coating film and an electrically conductive polymer layer and thereafter winds them together with the opposite cathode foil. However, it is difficult to wind the chemical conversion coating film and the electrically conductive polymer layer without damage to them and to accordingly form a wound electrolytic capacitor.
Further, the electrically conductive polymer layer can also be formed by impregnating the wound capacitor element with a monomer that is polymerized to become the electrically conductive polymer as well as an oxidizing agent. For example, Japanese Patent Laying-Open No. 11-186110 discloses a method of forming an electrically conductive polymer by impregnating a wound capacitor element with a monomer that is to serve as electrically conductive polymer and thereafter immersing the capacitor element in an aqueous solution of such an oxidizing agent as ammonium persulfate. Furthermore, Japanese Patent Laying-Open No. 2005-322917 discloses a method of forming a solid electrolyte by supplying a mixture of a precursor used for manufacturing an electrically conductive polymer and an oxidizing agent into a porous electrode coated with a dielectric layer, and exposing the porous electrode impregnated with the mixture containing the precursor and the oxidizing agent to a certain relative moisture so as to polymerize the precursor. These methods, however attach the oxidizing agent to the chemical conversion coating film on the anode and perform oxidation polymerization on the chemical conversion coating film. Therefore, the methods have a problem that the chemical conversion coating film is damaged and the chemical conversion coating film has a relatively large number of defects. Another problem is that the additional step of washing to remove the non-reacted monomer and oxidizing agent is required after the chemical oxidation polymerization, and thus the method is complicated.
Furthermore, Japanese Patent Laying-Open No. 05-144677 discloses a method of forming an electrically conductive polymer layer as described above by immersing a wound capacitor element in a solution in which such a soluble electrically-conductive polymer as polyaniline is dissolved, thereafter drying the capacitor element to remove the solvent. The method, however, has the problem that the electrically conductive polymer enters the inside of defects present in the chemical conversion coating film on the anode since the soluble conductive polymer is dissolved in the solvent, so that short-circuiting of the electrolytic capacitor is likely to occur and thus the voltage proofing properties are relatively poor.